It's the same with Mario. Even if the copyright on Super Mario Bros. expired today, Nintendo could claim trademark protection on the names and likenesses of Mario, Luigi, Princess Peach, Bowser, the Koopas, etc. Even the theme music. This has already been done. The "Tetris theme" is a public domain Russian folk song called Коробейники -- but The Tetris Company LLC owns a trademark on this song in the video game market.
Tetris is one of the most jealously guarded IPs in the world. Every aspect of Tetris is protected with copyrights and trademarks -- the board dimensions, the pieces, everything. Writing a game that plays like Tetris without a Tetris license is a good way to get sued and found liable. It's entirely possible for Nintendo to lock down protection of the Mario IP in virtual perpetuity in similar fashion, making copyright expiry moot for all but the most unimportant elements (say, the particular shapes of the clouds and horsetail plants in Super Mario Bros.).
Obviously someone calling for copyright expiration is calling for a sane interpretation (or reinterpretation, if you will) of trademark law.
Even under current law whether you can make a Micky Mouse cartoon probably has more to do with which judge your litigation is in front of than anything else.
We can debate whether Nintendo upholds that standard—I thought some of the Mario Party games were pretty atrocious—but at least the incentive exists.
I suddenly realized, when working on my mobile platform game, that adult me was developing the Mario game 12-year-old me always wanted to make. It just didn't have Mario in it or use the mechanics like specific power-ups unique to that series. But the platforming itself and, it turns out, things like how level layouts are stored, were pretty similar to Mario.
Kid me would be stoked for that bit of my future.
The idea of ideas being property is as human as anything.
The idea of ideas being property is, itself, as old as the idea of property.